Final answer:
All the variables listed - blood glucose levels, sodium levels, calcium levels, and pH levels - are controlled by negative feedback systems, which reduce fluctuations to maintain these variables within a normal range.
Step-by-step explanation:
The body variables listed in the question - blood glucose (sugar) levels, sodium levels, calcium levels, and pH levels - are all controlled by a negative feedback system. Negative feedback loops work to maintain homeostasis by reducing the fluctuation of a variable to keep it within a normal range. For instance, when blood glucose levels rise, the pancreas responds by releasing insulin, which facilitates the uptake of glucose by cells, thereby lowering the blood glucose levels back to normal.
Similarly, when red blood cells are sufficient in number, the production of erythropoietin is halted, which is a clear example of a negative feedback loop adjusting the stimulus in the opposite direction. Contrary to the incorrect statement in the question, blood calcium levels are also controlled by a negative feedback loop, where the levels are kept in check by mechanisms that reduce high calcium levels or elevate them when too low.
Therefore, the answer to the student's question is: 5) All of the above.